894
J . Phys. Chem. 1985,89, 894-896
Ultrafast Molecular Relaxation of Isolated Stilbene: Measurements by Picosecond Pump-Probe Techniques N. F. Scherer, J. W. Perry,+ F. E. Doany, and A. H. Zewail* Arthur Amos Noyes Laboratory of Chemical Physics,$ California Institute of Technology, Pasadena, California 91 125 (Received: January 11, 1985)
In this Letter, we report measurements of the picosecond relaxation time of trans-stilbene in SI under collisionless conditions. By accurate determination of the zero-time in our picosecond pump-probe experiments we obtained 7 = 3 f 0.5 ps, which is entirely consistent with our earlier measurements of IVR rates of stilbene in bulbs and beams. This establishes the molecular nature of the decay in these picosecond pump-probe multiphoton ionization (or mass spectrometry) experiments.
Introduction Picosecond fluorescence and pumpprobe techniques of molecules in supersonic jets have provided direct ways for probing intramolecular coherence and relaxation in isolated molecules.’ We have recently reported the results of picosecond time-resolved experiments of trans-stilbene under collisionless conditions in the vapor phase2 and in molecular beams.3 We used a pump-probe multiphoton ionization (MPI) method to monitor the ultrafast intramolecular vibrational redistribution (IVR) dynamics of this molecule. In these experiments a “UV” picosecond pulse prepared stilbene in a vibrational state in SI while a “visible” pulse probed the evolution of the initially prepared state by resonance-enhanced two-photon ionization. The ion signal (total or mass resolved) is detected as a function of the delay time between the pump (W) and probe (visible) pulses. With this arrangement we obtained (1) the biexponential character of the decay;2 (2) the isomerization rates as a function of excess vibrational energy? which agree with measurements made by fluorescence d e t e c t i ~ n and, ; ~ ~ ~(3) the polarization characteristics of the decay,3 which turned out to be consistent with recent theoretical findings.6 Because the fast component (=3ps) of the biexponential decay in stilbene is comparable in duration to the temporal width of our pulse, we could not provide a unique time constant for the decay even though we have made several experimental observation^^^^ that were consistent with this fast component being molecular in nature (and not due to some coherence artifact effects’ around zero time (t = 0) of delay): To obtain the actual molecular decay constant one must obtain the contribution of coherence to the observed signal and also address the possibility8 for other additional MPI processes that occur when the pump and probe pulses (temporally) overlap. This Letter reports experimental results which strongly confirm our earlier findings and establishes that a molecular decay is being observed in the stilbene pumpprobe MPI experiments. Specifically, we have determined the zero time and response function of our pump-probe arrangement by using the nonlinear optical process of difference frequency generation in a uniaxial ~ r y s t a l . ~ We conclude that the biexponential nature of the observed transient is consistent with the scheme for the time evolution of the excited states, as proposed earlier by Perry et al.,2 but the actual level structure is not established yet. Experimental Section The Pump-Probe Configuration. The experimental arrangement is similar to that described previously.2 In review, the laser system consists of a mode-locked Ar ion laser (Gaussian pulse, 10 nJ/pulse, 110 ps width) which synchronously pumps a dye laser containing a three-plate birefringent filter as a tuning element (all experimental results reported here were done at 3000-A pump ‘Present address: National Bureau of Standards, Gaithersburg, MD 20899. * Camille and Henry Dreyfus Foundation Teacher-Scholar. *Contribution No. 7141.
0022-3654/85/2089-0894$01.50/0
and 6000-A probe wavelengths). The dye laser pulses (1 nJ/pulse, 4 ps width) are amplified by the second harmonic of a Nd:Yag laser (3.5-11s pulse, 140 mJ/pulse, 20 Hz), which is electronically synchronized to the Ar ion mode locker driver, in a three-stage dye amplifier. Spatial filters isolate the three gain stages to facilitate wavelength tunability. The amplified pulses (0.5-1 mJ/pulse, 4 ps width) pass through a 2-cm KDP crystal to generate the second harmonic pump beam (see Figure 1). A dichroic mirror separates the fundamental from the UV light and directs the beams into the arms of a Michelson interferometer. The visible-beam arm contains a Soleil-Babinet compensator and a Glan-Thompson polarizer to allow adjustment of the relative polarization direction of the pump and probe beams. In this experiment, as in ref 2, the relative polarization is perpendicular. The UV arm contains only appropriate quartz neutral density filters. The beams are recombined with another dichroic mirror, which is complementary to the initial splitter, in a parallel but not collinear arrangement. Both beams are focused with a 25-cm plano-convex quartz lens (angle between the beams is a z o ) and are overlapped in a 20-pm pinhole at the UV beam waist. Once the overlap is established no further angular adjustments were made with the dichroic recombiner mount. The only change in the experimental system now consists of placing either the ionization cell (normal-set quartz window, nickel-chromium electrodes, 1.5 cm spacing) or a 1.5” LiI03 (or KDP) crystal at this position, as shown in Figure 1. To obtain the transients in the perpendicular pumpprobe configuration, the ionization cell was carefully situated such that the electrodes were placed at the overlap position. The pump beam intensity was adjusted to obtain good signal enhancement from the probe beam. Electrons are detected with a charge sensitive preamp (Ortec Model 109) which produces pulses proportional to the current. A boxcar averager (PAR Model 162 and Model 165 gated integrator) processes the chosen fraction of the waveform, and the analog output is fed into a ADC. The signal level is monitored as the variable delay line is scanned and stored in a multichannel analyzer (MCA) whose step advance (10-pm resolution) is synthronized to the stepper motor controller. The data are stored in and analyzed with a PDP 11/23 minicomputer. (!) For a review see, e.&, Felker, P. M.; Zewail, A. H. In ‘Applications of Picosecond Spectroscopy to Chemistry”; Eisenthal, K. B., Ed.; Reidel: Dortrecht, 1984; p 273. (2) Perry, J. W.; Scherer, N. F.; Zewail, A. H Chem. Phys. Lett. 1983, 103, 1. (3) Scherer, N. F.; Shepanski, J. F.; Zewail, A. H. J . Chem. Phys. 1984, 81, 2181. (4) Syage, J. A.; Felker, P. M.; Lambert, Wm. P.; Zewail, A. H.; Hochstrasser, R. M. Chem. Phys. Lett. 1982, 88, 266. ( 5 ) Syage, J. A.; Felker, P. M.; Zewail, A. H. J . Chem. Phys. 1984, 81, 4685, 4706. (6) Nathanson, D. M.; McClelland, G.M. J . Chem. Phys. 1984,82, 629. (7) See, e.&, Palfrey, S. L.; Heinz, T. F.; Eisenthal, K. B. “Ultrafast Phenomena IV”, Auston, D. H., Eisenthal, K. B., Ed.; Springer: Berlin, 1984; Chemical Physics 38, p 216. (8) Greene, B. I. in ref 7, p 308. (9) Yariv, A. “Quantum Electronics”; Wiley: New York, 1973; 2nd ed.
0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 6, 1985 895
Letters
I
MPI
i
I
VISIBLE ( l m J , 5ps) -20 I
I CROSS-CORRELATION
L
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. I
Figure 1. A schematic for the experimental arrangement used in obtaining the MPI and cross-correlation signals. The MPI "box" shows the ionization cell positioned at the UV beam waist. The cross-correlation 'box" illustrates the wave vector geometries for the two beams incident on the LiI03 (or KDP) crystal and the resultant difference frequency signal. SHG: second harmonic generation crystal, F neutral density filters, BS: dichroic beam splitter/recombiner, PD: photodiode.
-10
0
IO
20
30
40
TI ME ( ps) Figure 3. The simulations of a response function (curve a) which is a numerical best fit of the experimentally obtained cross correlation. (b) shows the convolution of the response function (a) with a biexponential decay (see text) where 7, = 3.0 ps and r2 = 250 ps. The i2 value was established in previous work while the i1 was varied to give the experimentally observed 2.5-ps peak shift. (c) is the transient response obtained by convoluting a single exponential decay with the cross-correlation function and adding the cross correlation response to reproduce the observed ratio of the fast/slow components of the decay.
time measurements were obtained under identical scanning conditions and plotted as such. Cross correlations were obtained before and after the stilbene transients to ensure that the experimental time zero did not change.
I
TIME (ps) Figure 2, The experimentally observed cross correlation (solid line through data points) and the perpendicular pump-probe polarization transient obtained with a three- late birefringent filter in the dye laser (Apump = 3000 A; APbc = 6000 and the 1.5-mm LiIO, crystals. The peak shift is indicated: 2.5 0.5 ps. The cross correlation and transient are normalized to the same peak amplitude for clarity in observing the peak shift.
k;,
Measurement o f t = 0 and Cross Correlation. The absolute time zero and pumpprobe cross correlation were obtained with the same experimental configuration used to obtain the transients, where the only change consists of removing the ionization cell from the beam overlap position and placing the LiI03 or KDP crystal in its stead. The difference frequency signal is obtained by adjusting the phase matching angle (not the beam overlap) and utilizing the orthogonality of the pump and probe beam polarizations. The phase matching condition (momentum and energy conservation) in this noncolinear geometry is such that the generated beam at frequency 2wl - wI is spatially separated from the probe beam. The generated beam was monitored with virtually zero background by a photodiode (visible light scattered off the crystal faces is the cause of the small constant background). The boxcar is again used to average the current-proportional pulse, and the processed signal is accumulated in the MCA. The variable delay is scanned over the same range to generate the field cross correlation (See Figure 2). Transients and the associated zero
Results and Discussion Figure 2 indicates that the absolute time zero, the peak of the cross correlation, and the peak of the ionization signal response are separated by 2.5 f 0.5 ps for the perpendicular pumpprobe polarization. To further substantiate these results (peak shifts) we mode locked the dye laser with a fine etalon in addition to the three-plate filter to obtain a cross correlation which was twice as long in duration. The shift between time zero and the peak of the MPI signal increased to 4.0 f 0.5 ps for perpendicular pump-probe polarizations. This shows that the shift is not caused by a different amount of dispersion caused by replacing the ionization cell with the LiI03 crystal. We have also attempted similar experiments for perpendicular pump and probe polarizations using a 1-cm KDP crystal. We obtain a shift of 3.0 f 0.5 ps when the dye laser was mode locked with a two-plate birefringent filter (the cross correlation is similar to that obtained in the three-plate experiment). Both crystals show a large t = 0 shift which is expected if a fast molecular decay is involved in the dynamics. To model the shifts we have performed computer simulations of convolutions of a response function with a biexponential decay. The response function used is a numerical best fit to the experimentally measured cross correlation.I0 The response function is convoluted with several biexponential decays of the form
+
f ( t ) = a1e-'/71 a2e-'/r2 where the slow decay component, 72, is always 250 ps (see ref 2). The fast component, T,,is varied from 0.05 to 10.0 ps and the relative ratio of fast-to-slow components, a 1 / a 2is, adjusted to reproduce the experimentally observed peak-to-long time signal level. Each fast decay produced a unique shift of the peak position relative to the experimental time zero ranging from 0.4 ps for T~ (10) In the above simulations, we have assumed that the experimental response function is the second-order correlation between the UV pump pulse and the visible probe pulse as given by ( I " " ( f ) I " ~ ~ ( f + r ) ) / ( I " " ( t ) I " ~The ~(t)). actual experimental response may be a higher-order correlation. In particular, a scheme where the pump is a single U V photon while the probe pulse is two visible photons yields a third-order correlation function ( I u v ( t ) I v , ~ ( ~ + r ) ) / (Iuv(t)Iwz(t)). Qualitatively, the higher-order correlations are sharper in time but the peak position remains the same. Thus the t = 0 determined by the difference frequency experiment is the actual time zero of the experiment.
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The Journal of Physical Chemistry, Vol. 89, No. 6, 1985
= 10.0 ps. In this experiment the = 0.05 ps to 4 . 5 ps for measured peak shift is 2.5 ps. The simulations reproduce this peak shift when r1= 3.0 f 0.5 ps. Figure 3 shows the response function and a simulation for T ] = 3.0 ps. Also shown in Figure 3 is a simulation of a transient using a single exponential decay ( r z = 250 ps) but with a ‘coherence artifact”. This curve (Figure 3c) is obtained by adding the appropriate fraction of the response function to the single exponential decay to obtain the experimental peak-to-long time signal level. The peak shift produced by this simulation is only 0.4 ps, which is not in agreement with the experimentally observed peak shift of 2.5 ps (as seen in Figure 2). In order to obtain the observed peak shift (see Figure 2 ) , a fast component of 3.0 f 0.5 ps was included in the simulations (see Figure 3b). The above results indicate that there is a real measurable shift due to a fast molecular decay in stilbene. As discussed by Perry et alezthis fast decay can be understood by invoking the idea that Franck-Condon factors between the neutral and the formed (by MPI) ions are different for the initially excited and redistributed vibrational states. In other words, following the initial state (S), preparation by the pump pulse molecular decays can lead to the population of other states (L) and the probe pulse ionizes the molecule from both S and L depending on the decay characteristic and the efficiency of ionization.* This idea of measuring dephasing of the initial state using MPI has actually been applied in this laboratory to other systems; picosecond transients have been observed in beams of pyrazine” and isoquinolineIz where in these cases the molecular decay times are much longer than the pulse width! An interesting question regarding the stilbene results is the following: what is the relationship between the observed decay time and the level structure of the states excited by the pulse? In a simple modelZ of energy redistribution one expects a quasi-biexponential decay with the fast component determined by the IVR rate and the slow component describing the lifetime@) of the states excited. However, as discussed elsewhere by one of us,33 this is only true for the simple case of an ‘optically active” mode coupled to a discrete set of “dark” states. In molecules like stilbene the spectral congestion is severe, even at rotational temperatures of
